pH sensors are used in clinics, laboratories and industrial factories since many biological and chemical reaction mechanisms are pH dependent. Conventional glass-type electrodes have been widely used; however, they still have certain disadvantages in specific applications. The glass rod sensor configuration is difficult to use for in vivo biomedical, clinical or food monitoring applications due to the brittleness of glass, size limitations and the lack of deformability. To achieve small sizes and robust design, ion-sensitive field-effect transistor (iSFET) pH sensors [1-5], optical fiber pH sensors [1, 6-11], hydrogel film pH sensors [12-14], and solid sate pH sensors [1, 15-18] have been proposed. iSFET sensors have power consumption concerns due to the field-effect transistor (FET) operational requirements [19]. Hydrogel film pH sensors utilize the physical properties of the pH-response swelling and shrinking polymer to measure resistance changes [12]. The sensor structure design and polymer layer fabrication process can be complicated and expensive [13]. Optical pH sensors also have power consumption issues due to the use of light sources. The system including optical devices could be expensive and unsuitable for implantation [1, 7-8, 10-11].
Various solid-state metal oxides have been investigated for pH sensing electrodes [1, 15] including PtO2, IrOx, RuO2, OsO2, Ta2O5, RhO2, TiO2 and SnO2 as the pH sensing films. The pH sensitivity, selectivity, working range, and hysteresis indicate sensing performance. IrOx, RuO2 and SnO2 have been demonstrated with more advantages in sensor performance for various applications [22]. RuO2 [18, 20] and SnO2 [21] show near Nernstian responses in wide pH ranges. However, SnO2 and RuO2 presented hysteresis and drift problems leading to potential calibration issues and unstable responses [20, 21]. Iridium oxide film (IROF) has performed outstanding stability over wide pH rages, rapid responses, less hysteresis and high durability, which have also been demonstrated at high temperature up to 250° C. [23].
There are different fabrication methods for IROF including sputtering deposition [23, 24], electrochemical deposition [25-29], thermal oxidation [23], and sol-gel [30-32] processes. The sputtering iridium oxide film (SIROF) deposition process is costly due to the target cost. The oxygen and argon pressure ratios, position of the target, deposition rate, and RF powers during the fabrication processes all affect the pH sensing parameters such as potential drifts and redox interference [22]. Anodic electrochemical deposition presents an economical way for iridium oxide thin film fabrication. The anodic iridium oxide thin film (AIROF) process is based on electrolysis of a solution containing iridium complexes. The iridium tetrachloride compound has been widely used as a deposition agent [26-29] such as the commonly used Yamanaka solution [26]. The pH value of the deposition solution, solution temperature and current density control affect the deposition efficiency [26-29]. A precise power supply system as potentiostate is required in the electro-deposition process for thickness and film quality control. For thermal oxidation process, it requires a high temperature ranging from 500 to 800° C. [17, 22]. The film made by thermal oxidation can be thicker than the AIROF with more stable potentials [22, 23]. However, the film surface has a tendency to crack after the high temperature treatment. The adhesion property of the cracked film then becomes an issue. The high temperature treatment also becomes a limitation during sensor fabrication, especially for the use of polymer and photoresist, which often can not survive at a temperature above 200° C. The sol-gel IROF deposition process has been demonstrated [32] with dip coating [32, 33] and heat treatment [31, 32] procedures. Sol-gel deposition provides a simpler and economical fabrication approach.
There is, therefore, a need for a cost efficient, simpler fabrication and lower power consumption, a metal-oxide pH sensor with deformability on a flexible substrate.